A variable reluctance rotation sensor of the prior art may typically comprise a rotatable toothed magnetic wheel and a stationary sensor unit having a permanent magnet for establishing a main flux loop. The toothed wheel and sensor unit define an air gap across which the main flux loop passes. The main flux loop also passes through a multi-turn electrical coil which generates an output voltage proportional to the rate of change of the main flux.
The toothed wheel comprises teeth, separated by slots, angularly spaced along its periphery. As the toothed wheel rotates, the air gap varies between a small air gap when the sensor unit is aligned with a tooth, and a larger air gap when the sensor unit is aligned with a slot. The air gap variations cause variations in the magnitude of the main flux loop passing through (linking) the electrical coil. The coil generates a sinusoidal output voltage signal having a maximum voltage signal each time the sensor unit is aligned with a tooth, and a minimum voltage signal each time it is aligned with a slot.
The amplitude of the signal in a variable reluctance rotation sensor of the prior art is dependant upon, among other factors, the size and strength (energy product) of the permanent magnet. Modern anti-lock braking and traction control systems require vehicle wheel speed data. This necessitates incorporation of a rotational sensor in a limited space, such as a vehicle wheel bearing. In order for the sensor to be small enough to meet packaging requirements, yet generate sufficiently high output voltage signals, expensive high strength rare earth permanent magnets are required, significantly adding to the cost of the rotational sensing device.
Additionally, the amplitude of the sensor signal decreases with rotational speed since the rate of change of flux due to air gap variation decreases as the tooth moves more slowly past the stationary member. Therefore, small inexpensive variable reluctance rotation sensors generating a high output at very low rotational speeds are not readily available.